Green Roofs by Lewis Williams

There are many forms of 'Green' or 'Living' roofs but they can be mainly divided into three groups categorised by the depth of growing medium (usually soil) upon the roof.  Each of these will be discussed in the second part of the article whilst the first part discusses the advantages and disadvantages of green roofs.

Advantages of green roofs :

• They provide an environment for wildlife that would not otherwise exist on a traditional roof.  The amount and extent of wildlife is dependant on the plants and design of the green roof - it has been proven that green roofs specifically designed to suit the local flora and fauna can create suitable habitats for the local wildlife.
• The use of green roofs aids towards reducing the energy consumptions of the building underneath.  This is because the green roof provides insulation in winter and conversely cools in the summer by absorbing the solar heat gains.
• Green roofs, in heavy rainfalls, can reduce the chance of local floods due to the amount of water that they can retain.
• The life of the existing roof material is prolonged due to the protection that the green roof can provide; resultantly the roof will be less likely to leak.
• For areas with high noise pollution, green roofs can aid towards reducing noise, for the building underneath it.
• The water that does run-off the green roof can be collected and used to water the green roof (if necessary) or other parts of the garden.
• Green roofs are adaptable because they can be installed to a range of roofs including ones with a pitch of up to forty degrees, but the disadvantage of such a roof will incur higher design and installation costs.
• And finally one of the most important points, is that green roofs greatly improve the appearance of the roof.  Looking onto the roof means that instead of looking at the bleak asphalt or traditional roof surface there is an oasis full of life, whether that is just plants with/without the wildlife.
  

Outdoor building with sedum roof at private garden:
'The Old Zoo'  located in the Ribble Valley, near Whalley.

Disadvantages of green roofs :

• Structural aspects of the roof may need to be looked at and in some cases strengthened to hold the weight of the green roof, the degree of structural integrity will be dependant on the type of green roof.
• The existing waterproofing layer of the roof may need to be replaced but this can be easily done during the installation of the green roof.
• They do cost more than traditional roof materials, however the money saved from their use can be greater than their cost and maintenance.  The price per square metre is dependant on the green roof system (and supplier) and as with any plant buying it is dependant on the impact required upon installation - i.e. the cost of pre-grown sedum mats is higher than sedum plugs which take time to grow and cover the roof.  Plants can be grown from seeds, cuttings, plugs or mats, so the size of plant is dependant on the price.
• They do require maintenance – the amount is dependant on the type of green roof (and plants) but as a bare minimum annual maintenance will be required to remove unwanted self-seeding plants.
  

A typical green roof system comprises of the following :

• The plants which is dependant on the type of green roof system.
• The growing medium which can comprise of mats of a soil mixture.
•  Filter layer which allows the water to pass through it but not soil or substrate which would block the drainage layer.
• A drainage layer that determines the amount of water that is stored on the roof and allowed to drain off.
• Root barrier layer which stops the roots damaging the roof.
• Insulation and Waterproof layer which are components of the existing roof and can be renewed prior to installation.
• System can vary from the above dependant on the supplier and manufacturer and there are systems available that only use sustainable and recyclable materials, which the author recommends.
  
   

The three main types of green roofs are:

Extensive roofs - ones where the depth of soil is shallow between 0-15cm.  The plant species are limited to a small number - namely sedums and lichens and mosses.  However, it is the most common type of green roof because of its low maintenance and as is the lightest system.  Types of plants that can grow are sedums, mosses, lichens, short wildflower varieties and a range of low-growing drought tolerant grasses, perennials and alpines.

Sedums growing in a 'green' roof environment

Semi-intensive roofs - contains a medium depth of soil which is more suitable for growing a wider range of plants including wildflower meadows, hardy sub-shrubs and medium perennials, grasses, bulbs and drought tolerant annuals.

Intensive roofs - contains a deep depth of soil meaning that it is practically a raised garden as the suitable plants range includes lawns, shrubs and trees.  This type of roof is uncommon and not very practical in domestic situation.

In summary, I see green roofs as an effective way at brightening up the usually bland and wasted space that is a roof.  As shown they do have many positive environmental advantages which will become evermore important in this environmentally conscious society.  Green roofs are still a relatively new market within the UK when compared to other countries such as Germany, however with increasing popularity and awareness of such a product then the suppliers will grow and the prices should drop.


Lewis Williams (April 2008)

References and sources of further information :

• Gedge, D., Dunnet, N., Grant, G. and Jones, R. (2007), Living Roofs, Catalogue code NE30 Natural England www.naturalengland.org.uk.
• Chartered Institute of Building Service Engineers London (2007), Green Roofs, CIBSE Knowledge Series.
• A company that designs, supplies, installations and maintains of lightweight roof plating systems - www.greenroof.co.uk.
• Supplier and grower of green roofs systems - www.liveroof.net.
• Independent UK resource for Green roof information - www.livingroofs.org.